Conventionally, it has been known to use phosphors that absorb visible light rays in a short wavelength region such as blue light rays and ultraviolet rays and convert these light rays in wavelength to visible lights with longer wavelengths such as red and green lights and obtain visible light such as white light by combining these phosphors.
In particular, as a light source of visible light rays in the short wavelength region or ultraviolet rays, known is a semiconductor light-emitting element, for example, a gallium nitride (GaN)-based blue light-emitting diode or the like. Moreover, a light-emitting element that is constructed in combination with a phosphor serving as a wavelength converting material to emit visible light such as white light features a small amount of power consumption and a long lifespan, and has therefore received attention as a light-emitting source of an image display device or a lighting system in recent years.
Moreover, for this light-emitting element, as described in, for example, Japanese Laid-Open Patent Publication No. 10-242513, a phosphor serving as a converting material absorbs visible light in the blue region emitted by a GaN-based blue light-emitting diode to emit yellow light, and emits white as a result of mixing with blue light of the light-emitting diode not absorbed in this phosphor.
However, this white light emission lacks color rendering properties as this is formed of light of a blue color and a yellow color being a complementary color of this blue color.
In addition, as a light-emitting element improved in color rendering properties of white light formed of light of these blue and yellow colors, as described in, for example, Japanese Laid-Open Patent Publication No. 2002-60747, known is one that forms white light by combination of a blue light-emitting diode, a green-emitting phosphor, and a red-emitting phosphor or an orange-emitting phosphor.
However, a stable phosphor that efficiently emits red or orange light upon excitation by irradiation of blue light is still as yet unknown. For example, an alkaline-earth metal sulfide phosphor by activation of divalent europium (Eu2+) emits a reddish color, but easily undergoes a chemical change, and may produce a harmful sulfur compound such as a hydrogen sulfide gas. In particular, when a light-emitting diode is formed by this alkaline-earth metal sulfide-based phosphor, there is a problem such as corrosion of a surrounding material and the like.
In addition, as a phosphor that emits an orangish color, as disclosed in, for example, Japanese Laid-Open Patent Publication No. 2006-36943 or Japanese Translation of International Application No. 2006-514152, known is an alkaline-earth metal silicate phosphor by activation of divalent europium (Eu2+) such as Sr3SiO5:Eu. However, this alkaline-earth metal silicate phosphor is not sufficient in luminance and has a problem that it is not easy to secure high luminance.
The present invention has been made in view of such problems, and it is an object of the present invention to provide an orange-emitting phosphor that emits at a high luminance.